This revised application for our new Program Project Grant brings together a group of investigators who are experts in the areas of molecular and cellular immunology and human and mouse genetics. We propose to study the role of single genes in the pathogenesis of X-linked Lymphoproliferative syndrome (XLP), Common variable immunodeficiency (CVID), Omenn syndrome and Severe Combined Immunodeficiencies (SCID) in an application, entitled: "Primary Immunodeficiencies Affecting Specific Stages of the Adaptive Immune Response" Because of successful preliminary analyses of patient materials, exciting findings in genetically altered mice, this application seeks to define how mutations in the SH2D1A, SH2D1B/C, TACI, RAG-1/2, DNA Ligase IV or Cernunnos genes affect T and B cell development and T cell dependent and/or T cell-independent immunoglobulin responses. We will use our recently acquired insights into the causes of these diseases in the following four interlinked projects and an Administrative Core: Project #1 Role of the SAP (SH2D1A) gene in T cell-dependent antibody responses. Cox Terhorst, Beth Israel Deaconess Medical Center. Project #2 Role of in TACI mutations in Common Variable Immunodeficiency Raif Geha, Children's Hospital of Boston. Project #3 Gene knock-in models for Omenn syndrome and leaky SCID. Luigi Notarangelo, Children's Hospital of Boston. Project #4 Mouse Models of Severe Combined Immunodeficiencies. Fred Alt, Children's Hospital of Boston. Core A Administrative Core Cox Terhorst, Beth Israel Deaconess Medical Center. The outcomes of proposed studies should lead to better understanding of the complex and often alternate disease manifestations that are caused by mutations in a single gene. The results of these studies should suggest therapeutic strategies that can be applied to these PID patients and may unravel molecular and cellular mechanisms that are generally involved in immune dysregulation, autoimmunity and cancer. PROJECT 1: Role of SAP (SH2D1A) gene in T cell-dependent antibody response (Cornelis Terhorst) PROJECT 1 DESCRIPTION (provided by applicant): X-linked lymphoproliferative syndrome (XLP) is characterized by three prevalent phenotypes: 1) Fulminant Infectious Mononucleosis, 2) malignant B cell lymphoma and 3) acquired hypogammaglobulinemia in the absence of infectious mononucleosis. As the three major phenotypes of XLP are found within one family harboring the same mutation, genetic background and/or environmental factors must play a role in the pathogenesis of the disease. FIM patients mount a vigorous, uncontrolled polyclonal expansion of T and B cells, inevitably leading to death by hepatic necrosis and bone marrow failure. A major cause of XLP in humans is a defect in the SH2D1A gene, which encodes SAP (SLAM Associated Protein), a single free SH2- domain protein that controls distinct key signal transduction pathways in CD4 and CD8 T lymphocytes, NK cells, platelets and probably a subset of B cells. SAP functions as an adapter, which bridges the cytoplasmic tail of six members of the SLAM receptor family to signal transduction pathways, e.g. Fyn and other tyrosine kinases. Previous studies show that several arms of the acquired immune responses are affected in XLP patients and SAP-deficient mice and that the SAP related molecules EAT-2A and -2B subserve similar functions. Our general hypothesis is that SAP and EAT-2A/B control partially overlapping mechanisms that are critical for the control of humoral immunity. The experiments proposed in this application will test the hypothesis that #1) distinct SAP-/- CD4+ T cell subsets and/or NKT cells contribute to the dysgammaglobulinemia of SAP-/- mice, #2) disruption of the SAP gene affects the subsets of follicular B cells, which participate in humoral responses and in the germinal center reaction and #3) EAT-2A/B and SAP interact in the control of antibody responses. Together these experiments should clarify the role of SAP and EAT-2 in T cell dependent B cell responses and why the absence of SAP functions causes dysgammaglobulinemia. The results of these studies should suggest therapeutic strategies that can be applied to XLP patients.